Ballistic shape screwhead socket

ABSTRACT

Square socket-type head for screw fasteners, and punch for forming same, in which the socket has four equal sides each of which has an upper slightly inwardly inclined flat portion which merges without discontinuity at its lower end with a lower concave bottom portion.

a y l] ited States Patent [151 3,64,175

arclay 1 Feb. 8, 1972 [54] BALLISTIC SHAPE SCREWHEAI) Relcrencmi (flu-d SOCKET v UNI'I'HI) S'IA'IHS IA'IHN'I'S 'l t l 7 All N l [I H H 2,397,2l6 3/1946 Stellin ..s5/45 2,9 I 4,984 [2/1959 Ansingh ..85/ ['73] Assignee: The Steel Company of Canada, Limited, ,3 2/1965 Johnson 5 Hamilton, Ontario, Canada 3,187,790 6/ l965 Wing ..85/45 [22] Filed: 1968 I Printary ExaminerEdward C. Allen [21 Appl.No.: 768,863

[57] ABSTRACT [52] [1.5. CI ..85/45, 72/476 s v 4 t quare socket-type head for screw fasteners, and punch for [5 1 1 Cl 23/00 forming same, in which the socket has four equal sides each of which has an upper slightly inwardly inclined flat portion which merges without discontinuity at its lower end with a lower concave bottom portion.

[58] Field of Search... ..85/45 6 Claims, 14 Drawing Figures BALLISTIC SHAPE SCREWHEAD SOCKET BACKGROUND OF THE INVENTION This invention relates to screw fastening devices having heads which include a square recess or socket for engagement with a driving means, and to punches useful for forming such a recess.

Hitherto a number of different types of screw fastening devices having square or recess socket driver engagement means have been .made available commercially. Among designs which have been extensively used are those which disclose square recess screws in which the bottom of the recess is formed in the shape of a pyramid or cone, respectively. Such designs, and all other previous designs of this type of driving recess have been characterized by two features which give rise to substantial disadvantages. In the first place, the lower portion of the socket makes a definite, more or less sharp" intersection or discontinuity where it joins the upper, substantially flat and vertical sides which are used to engage the driving means. Secondly, the lower portion in itself terminates in a more or less pointed configuration. The punch used to produce such a socket has of course corresponding characteristicsi.e., definite line of intersection between the flat vertical sides and the lower portion, and a more or less pointed tip.

These characteristics in prior socket and punch design give rise to a number of serious deficiencies. Considering for instance the pyramidal or conical point forms, it is apparent that as a punch of the shape in question forms an impression in the head of a screw blank, there is a most uneven distribution of force. A pyramidal punch has unidirectional forces on each plane face surface of the bottom portion, with .the result that metal flow cannot occur uniformly as the punch moves into the blank. Such force stops abruptly at the junction of the pyramidal portion and the square upper section of the socket. A conical punch advancing into a blank creates a comparable reaction to a pyramidal punch, as explained above. A conical punch and a pyramidal punch have a nonunifonn direction of force at the junction of the conical lower portion and the sidewalls of the socket, and in both cases there is sudden reduction of force at the junction of the face angle and the side angle. These substantial variations in the displacement force imposed upon the screw blank by the punch prevents adequate control of metal displacement as the punch proceeds in its forward direction. The result is that the stress responsible for moving the metal back from the point of the punch continues to displace the metal in a direction comparable to that established by the angle of the cone or pyramid in question so long as the main or forward force is maintained. This overshoot or extra unwanted movement of the metal tends to cause a displacement of the blank stock of the screwhead away from the sidewalls of the socket punch, thus giving rise to a void or fall-away" in the finished product. Such a result, of course, prevents optimum engagement of the socket with the driving tool along its full effective length and at the same time gives rise to substantial variations in stress along the faces of the socket in a way which reduces the driving torque which it is capable of transmitting to the screw. 5

Apart from the deficiencies mentioned above which relate to the socket head screw itself, the prior punch designs used in manufacturing the aforementioned type of screws suffer from rapid wear because of the more or less pointed lower end and the sharp intersection between the lower portion of the die and the flat sides. Moreover, the fact that there is substantial fall-away in prior socket head screw designs has contributed to reduced driving-tool life since that part of the tool which enters the socket does not engage it in a uniform manner from top to bottom.

SUMMARY OF THE INVENTION I have found that the disadvantages associated with prior socket-type screw fasteners can be overcome by providing a screw fastening device which has a socket centrally disposed about the longitudinal axis of the screw, the socket having a plurality of equal sides, each of which has an upper flat portion which extends downwardly and slightly inwardly, and which merges into a lower, concave portion that extends downwardly and inwardly to the bottom of the socket. The curve defined by the intersection of said Ioiiver portion of a side of thesocket with any plane which includes said axis has, at its point of of merger with said upper flat portion, a tangent which is substantially coincident therewith and, at the point of intersection with said axis, a tangent which is substantially perpendicular thereto. Preferably the cross section of the socket, in a progression from top to bottom of parallel planes perpendicular to the axis, is that of a regular polygon or square in the region of the upper portions of the socket, whereas it is that of a generally similar polygon or square having rounded comers in the region of the lower portions of the socket. Such a socket (or the punch used for producing the same)-may conveniently be'referred to as being of ballistic shape. As will be apparent from the description just given, the bottom of the socket and the punch is of a rounded, blunt form, and the lower portion merges smoothly, at all points around its periphery, into the flat sides. By such an arrangement much more adequate control is effected over displacement of the stock of the screw blank head as the punchadvances into it, thus resulting in substantial reduction, if not elimination, of fall-away" and a much more even distribution of stress in the region of the socket formation. Consequently, improved quality of the finished screwhead results and a greater utilization of material is efiected. Also as a result of the smooth metal flow, uniform work hardening and adherence to tool form on the impression, the metal being displaced in order to form the screwhead is less likely to rupture. Moreover the ballistic shape of the punch contributes substantially to tool life and the optimum characteristics of the socket thus formed permit much better engagement between the drive means and screw head whereby driver life is extended as well. The ballistic shape punch is the subject of my copending application, Ser. No. 180,713 filed Sept. 15, 1971.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be described in connection with the attached FIGURES of drawings in which:

FIG. 1 is a top plan view of a screw driving socket made in accordance with the invention, together with a full vertical cross section (FIG. 1A) taken along the line A-A, and vertical half cross sections (FIGS. 13, 1C and 1D) taken respectively along lines BB, C-C and DD of FIG. 1;

FIGS. 2 to 9 are horizontal half cross sections respectively taken along the planes defined by the contour lines of corresponding-number in FIG. 1, and the section lines of corresponding number in section A; and

FIG. 10 is an isometric view of a punch made in accordance with the invention showing contour lines 2 to 9 which define planes in which the half cross sections of the punch are those of FIGS. 2 to 9 respectively.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1 the upper part is a top plan view of a square socket generally represented at 20 and formed, in accordance with the invention, in the head 21 of a screw blank. In this Figure, the usual chamfer for facilitating entry of the driving device into the socket is shown at 22 although such charnfer forms no part of the present invention which relates only to the form of the recess or socket below its bottom edge 23.

As is apparent from FIG. 1, the recess 20 is symmetrically disposed about the longitudinal axis 24 of the screw. The profile of each of the four equal sides of the socket is best shown in the sections labeled A, B, C and D which are taken on the lines A-A, BB, C-C and DD which are respectively disposed at 90, 75, 60 and 45 degrees from the vertical line 25. Referring now to the full sections A, it will be observed that the socket comprises four equal sides each of which has an upper flat portion 26 which extends downwardly and slightly inwardly at an angle 27 (preferably between 33 and 3 9E") and which at its lower end 28 merges smoothly into a lower, concave portion 29 that extends both downwardly and inwardly to the bottom 30 of the socket in such a way that the curve defined by the intersection of said lower concave portion of a side of the socket with any of the planes (e.g. those on line A-A, BB, C-C and D-D) intersecting a side of the socket and passing through the axis 24 has, at its point of merger with said upper flat portion a tangent which is substantially coincident therewith and, at the point of intersection with said axis, a tangent which is substantially perpendicular thereto. More particularly, it will be noted that the bottom 30 of the socket is substantially rounded and the upper flat portion 26 and the lower curved portion 29 merge smoothly at 28 into one another without any apparent line of intersection of discontinuity.

As will be apparent from an examination of FIG. 1, cross sections of the sockets taken on planes perpendicular to the axis 24 between the lower edge 23 of the chamfer and line 28 will be square in shape, and of a gradually diminishing crosssectional area corresponding to the angle 27 at which the sides of the socket converge slightly inwardly. Below the line 28, in the region of the lower, concave portion 29, the cross section of the socket in planes represented by the lines 2 to 9 respectively, are represented by the half cross-sectional views of FIGS. 2 to 9. As is apparent from the latter, the first cross section given by FIG. 2, which is immediately below the line 28, is that of a square with very slightly rounded or suppressed corners. As succeeding cross sections are taken along the lines 3 to 9 respectively, in a progression from the top to the bottom of the lower concave portion 29, it will be observed that the sections are also of generally square configuration, but with increasingly rounded comers and decreasing cross-sectional area until the lowest cross section shown (i.e., that of FIG. 9) approximates that of a circle.

Referring now to FIG. 10, a punch 35 is there illustrated which is adapted to form the socket 20 shown in FIG. 1. The punch 35 has four equal sides each of which has an upper flat portion 36 which extends downwardly and slightly inwardly at the angle 27, and which merge smoothly at their lower ends into a lower convex portion 37 which extends both downwardly and inwardly to the bottom or tip 38 of the punch in such a way that there is substantially no discontinuity in the junction between the upper and lower portions 36, 37 whereby (as was the case with the socket itself) there is no discontinuity in the curve which would be defined by the intersection of a side of the punch with any plane which includes the axis 24 passing therethrough.

What I claim is:

1. In the head of a screw fastening device, a socket centrally disposed about the longitudinal axis of the screw, said socket being formed of a plurality of equal sides each of which has an upper flat portion which extends downwardly and slightly inwardly and which merges smoothly into a lower generally concave portion that extends downwardly and inwardly to form a bottom of the socket symmetrically disposed about said axis whereby the curve defined by said lower portion of a side of the socket with any plane which includes said axis has, at its point of merger with said upper flat portion, a tangent which is substantially coincident therewith and, at the point of intersection with said axis, a tangent which is substantially perpendicular thereto, and wherein the cross section of the socket is, in a progression from top to bottom of parallel planes perpendicular to said axis, that of a regular polygon in the region of said upper portions, and that of a generally similar polygonal shape having rounded comers in the region of said lower portion.

2. A screw fastening device as claimed in claim 1 wherein the socket has four equal sides and a square cross section.

3. A screw fastening device as claimed in claim I in which the sides of the said upper portion are inwardly inclined towards the axis between about 3 and 3%.

4. A screw fastening device as claimed in claim 1 wherein the cross section of said socket is, in a progression from top to bottom of parallel planes perpendicular to said axis, that of a regular polygon of slightly diminishing size in the region of said upper portions, and in the region of said lower portions, that of a generally similar polygonal shape having progressively more rounded comers and decreasing area whereby in planes adjacent the bottom of the socket said cross section is of approximately circular configuration.

5. A screw fastening device as claimed in claim 4 wherein the socket has four equal sides and a square cross section.

6. A screw fastening device as claimed in claim 4 in which the sides of the said upper portion are inwardly inclined towards the axis between about 3 and 392. 

1. In the head of a screw fastening device, a socket centrally disposed about the longitudinal axis of the screw, said socket being formed of a plurality of equal sides each of which has an upper flat portion which extends downwardly and slightly inwardly and which merges smoothly into a lower generally concave portion that extends downwardly and inwardly to form a bottom of the socket symmetrically disposed about said axis whereby the curve defined by said lower portion of a side of the socket with any plane which includes said axis has, at its point of merger with said upper flat portion, a tangent which is substantially coincident therewith and, at the point of intersection with said axis, a tangent which is substantially perpendicular thereto, and wherein the cross section of the socket is, in a progression from top to bottom of parallel planes perpendicular to said axis, that of a regular polygon in the region of said upper portions, and that of a generally similar polygonal shape having rounded corners in the region of said lower portion.
 2. A screw fastening device as claimed in claim 1 wherein the socket has four equal sides and a square cross section.
 3. A screw fastening device as claimed in claim 1 in which the sides of the said upper portion are inwardly inclined towards the axis between about 3* and 3 1/2 *.
 4. A screw fastening device as claimed in claim 1 wherein the cross section of said socket is, in a progression from top to bottom of parallel planes perpendicular to said axis, that of a regular polygon of slightly diminishing size in the region of said upper portions, and in the region of said lower portions, that of a generally similar polygonal shape having progressively more rounded corners and decreasing area whereby in planes adjacent the bottom of the socket said cross section is of approximately circular configuration.
 5. A screw fastening device as claimed in claim 4 wherein the socket has four equal sides and a square cross section.
 6. A screw fastening device as claimed in claim 4 in which the sides of the said upper portion are inwardly inclined towards the axis between about 3* and 3 1/2 *. 